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The module system internally transforms module syntax into definitions. This always happens internally.
:::
It is possible to create first class definitions which are not transformed _again_ into definitions by the module system.
Usually the file location of a definition is implicit and equal to the file it came from.
However, when manipulating definitions, it may be useful for them to be completely self-contained (or "free-floating").
A free-floating definition is created with `mkDefinition { file = ...; value = ...; }`.
Preserving the file location creates better error messages, for example when copying definitions from one option to another.
Other properties like `mkOverride``mkMerge``mkAfter` can be used in the `value` attribute but not the other way around.
The following shows an example configuration that yields an error with the custom position information:
```nix
{
_file="file.nix";
options.foo=mkOption{
default=13;
};
config.foo=lib.mkDefinition{
file="custom place";
# mkOptionDefault creates a conflict with the option foo's `default = 1` on purpose
# So we see the error message below contains the conflicting values and different positions
value=lib.mkOptionDefault42;
};
}
```
evaluating the module yields the following error:
```
error: Cannot merge definitions of `foo'. Definition values:
- In `file.nix': 13
- In `custom place': 42
```
To set the file location for all definitions in a module, you may add the `_file` module syntax attribute, which has a similar effect to using `mkDefinition` on all definitions in the module, without the hassle.
